Add a default reactive cache key

Aligns with the other reactive cache options by
providing a default that can be overridden if necessary.

app/models/clusters/cluster.rb

@@ -8,7 +8,6 @@ module Clusters
     include ReactiveCaching
     self.table_name = 'clusters'
-    self.reactive_cache_key = -> (cluster) { [cluster.class.model_name.singular, cluster.id] }
     PROJECT_ONLY_APPLICATIONS = {
       Applications::Jupyter.application_name => Applications::Jupyter,

app/models/clusters/platforms/kubernetes.rb

@@ -11,7 +11,6 @@ module Clusters
       RESERVED_NAMESPACES = %w(gitlab-managed-apps).freeze
       self.table_name = 'cluster_platforms_kubernetes'
-      self.reactive_cache_key = ->(kubernetes) { [kubernetes.class.model_name.singular, kubernetes.id] }
       belongs_to :cluster, inverse_of: :platform_kubernetes, class_name: 'Clusters::Cluster'

app/models/concerns/prometheus_adapter.rb

@@ -6,7 +6,6 @@ module PrometheusAdapter
   included do
     include ReactiveCaching
-    self.reactive_cache_key = ->(adapter) { [adapter.class.model_name.singular, adapter.id] }
     self.reactive_cache_lease_timeout = 30.seconds
     self.reactive_cache_refresh_interval = 30.seconds
     self.reactive_cache_lifetime = 1.minute

app/models/concerns/reactive_caching.rb

@@ -10,8 +10,6 @@
 #    class Foo < ApplicationRecord
 #      include ReactiveCaching
 #
-#      self.reactive_cache_key = ->(thing) { ["foo", thing.id] }
-#
 #      after_save :clear_reactive_cache!
 #
 #      def calculate_reactive_cache
@@ -89,6 +87,8 @@ module ReactiveCaching
     class_attribute :reactive_cache_worker_finder
     # defaults
+    self.reactive_cache_key = -> (record) { [model_name.singular, record.id] }
+
     self.reactive_cache_lease_timeout = 2.minutes
     self.reactive_cache_refresh_interval = 1.minute

spec/models/concerns/reactive_caching_spec.rb

@@ -232,4 +232,17 @@ describe ReactiveCaching, :use_clean_rails_memory_store_caching do
       end
     end
   end
+
+  describe 'default options' do
+    let(:cached_class) { Class.new { include ReactiveCaching } }
+
+    subject { cached_class.new }
+
+    it { expect(subject.reactive_cache_lease_timeout).to be_a(ActiveSupport::Duration) }
+    it { expect(subject.reactive_cache_refresh_interval).to be_a(ActiveSupport::Duration) }
+    it { expect(subject.reactive_cache_lifetime).to be_a(ActiveSupport::Duration) }
+
+    it { expect(subject.reactive_cache_key).to respond_to(:call) }
+    it { expect(subject.reactive_cache_worker_finder).to respond_to(:call) }
+  end
 end